In order to improve fuel economy during low load conditions, some engines may be configured to operate in a selective cylinder deactivation mode where one or more cylinders of the engine are deactivated via disabling of intake and/or exhaust valve actuation, interruption of fuel injection, and/or disabling of spark ignition to the deactivated cylinders, for example. During operation in the selective cylinder deactivation mode, also referred to as “skip fire,” the total engine fuel amount may be redistributed to the fired cylinders, increasing per-cylinder load and reducing pumping work, thus increasing fuel economy and improving emissions. The cylinder(s) selected for deactivation may change with each engine cycle, such that a different cylinder or combination of cylinders is deactivated per engine cycle. Further, the number of cylinders deactivated per engine cycle may change as engine operating conditions change.
The inventors herein have recognized that during skip fire operation, valve deactivation/reactivation mechanisms may not be fully reliable. This may lead to unintended combustion events in cylinders scheduled to be skipped and/or unintended skipping of cylinders scheduled to be fired. Unintended firing or skipping of cylinders may cause undesired torque changes, NVH issues, degraded emissions, and/or other problems.
In light of the above issues, the inventors herein have devised an approach to maintain robustness of a skip fire strategy. One example method comprises: for a given engine cycle of an engine operating in a skip fire mode, selecting a number of cylinders of the engine to skip based on engine load and setting a commanded firing order of non-skipped cylinders of the engine, where the commanded firing order includes scheduling at least a first cylinder to be fired and at least a second cylinder to be skipped. The method further includes determining if combustion occurs as commanded in the first cylinder. If combustion does not occur, the commanded firing order is adjusted to fire the second cylinder of the engine. In one example, combustion may be detected based on feedback from an ionization sensor.
Similarly, combustion may sometimes occur in both the first cylinder and the second cylinder, although the second cylinder was intended to be skipped. In this case, the commanded firing order is adjusted to skip a later cylinder in the firing order which was originally planned to fire.
In this way, the commanded firing order of the engine may be dynamically updated in response to unintended combustion events, including combustion occurring in cylinders scheduled to be skipped and lack of combustion in cylinders scheduled to be fired.
The present disclosure may offer several advantages. For example, by updating the firing order to compensate for unintended cylinder events during skip fire, desired torque may be maintained, even if valve actuation does not occur as commanded.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.